In sonoluminescence, the bubbles in a liquid emit light when they are forced to expand and collapse by sound waves. Physicists believe that the pressures and temperatures inside the collapsing bubbles could be high enough to initiate nuclear reactions. If achieved, such “bubble fusion” could lead to a new clean energy source.

In a controversial paper published in Science in March 2002 Taleyarkhan and co-workers described how they had used high-energy neutrons to create tiny bubbles of gas in a beaker of acetone in which the hydrogen atoms had been replaced by deuterium (D). Taleyarkhan, who was then based at the Oak Ridge National Laboratory, claimed that the temperature inside the collapsing bubbles was in excess of a million degrees – high enough for two deuterium nuclei to undergo a fusion reaction (Science 295 1868). DD fusion reactions can produce a helium-3 nucleus plus a neutron, or a tritium nucleus and a proton.

However, the results were questioned by many researchers in the field. Now, Taleyarkhan says his team has repeated the experiment with more sensitive detectors. “A fair amount of very substantial new work has been conducted,” he said in a press release issued by Purdue. “And this time I made a conscious decision to involve as many individuals as possible – top scientists and physicists from around the world and experts in neutron science.”

As before, the team claims to detect tritium as well as neutrons with the characteristic energy for DD fusion reactions. Moreover, the fusion products are not observed in experiments with ordinary acetone. Taleyarkhan says that chances of the result being due a phenomenon other than fusion have been reduced from 1 in 100 to 1 in 1011.

Michael Saltmarsh of Oak Ridge says he is “intrigued but sceptical” about the new work. “Unlike their Science paper, most of the background notes and supporting information seem to be correct but there are still some puzzling inconsistencies,” he told PhysicsWeb. “In particular, the estimated neutron detection efficiency is still an order of magnitude too low. While better than the Science article, the difference would produce a mismatch between the reported neutron and tritium yields.”

“Thermonuclear sono-fusion may not be impossible,” says Willy Moss of the Lawrence Livermore National Lab, “but more tests need to done. Personally, I would like the results to be real, but I believe that the nature of these claims requires absolute proof.”

“When a startling new discovery is announced, it is the responsibility of the authors to lay things clear,” adds Aaron Galonsky of Michigan State University. “Taleyarkhan and co-workers have not done that well enough for me to be able to say whether they have seen nuclear fusion in a bottle of acetone. With two million 14 MeV neutrons per second injected into the room where the experiment was performed, there are opportunities for error in detecting the much rarer, lower-energy sonoluminescent neutrons.”